Neutron skin measurement of tin isotopes

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Abstract

Heavy atomic nuclei are thought to have proton and neutron radial distributions
which have different extents. This difference is usually quantified in
terms of a neutron skin (rnp), defined as the difference between the root mean
square radii of the neutron and proton radial distributions (rnp = rn - rp). The
nature, or even existence, of the neutron skin is currently not well established for
many nuclei. Different nuclear theories give different predictions for the neutron
skin thickness ranging for a typical heavy nucleus from 0.05 to 0.35 fm. Accurate
measurement of the properties of the neutron skin would be a powerful constraint
to differentiate between models of nuclear structure, improving our knowledge of
the basic Equation Of State (EOS) for neutron rich matter. Particularly, the
rate at which the neutron skin thickness changes across an isotopic chain of
nuclei gives a tight constraint on the EOS and is also amenable to experimental
determination with small systematic error. Improving our knowledge of the EOS
for neutron rich matter is a crucial step towards gaining a deeper understanding
of nuclear structure and nuclear matter in general. These results will also impact
our knowledge of compact astrophysical objects such as neutron stars. This thesis
describes the first measurement of neutron skin thicknesses along an isotopic chain
using an electromagnetic probe. The neutron skin is measured through the study
of the coherent photoproduction of neutral π mesons emitted from nuclei.
This experiment was carried out in the A2 hall of the MAMI facility in Mainz,
Germany in October 2012. The incident photon beam comprised of energy tagged
photons in the range of Eγ=150-800 MeV with an intensity of 10⁸ photons per second. Experimental data was obtained for three different tin targets, 116Sn,
120Sn and 124Sn. The products from the resulting photoreactions were measured
in the Crystal Ball detector and in the TAPS calorimeter systems, with track and
particle identification information for charged particles provided by a multi wire
proportional chamber (MWPC) and a particle identification detector (PID).
The experiment provides the first information on the evolution of the neutron
skin thickness along an isotopic chain using an electromagnetic probe. The results
are compared with a range of theoretical models and previous data from strongly
interacting probes. The new data will provide an important new experimental
constraint on the basic properties of the EOS in atomic nuclei.